1. Field of the Invention
The present invention relates to a preparation process of a copolymer suited for use as a film forming polymer such as resist polymer, polymer used for an antireflective film and polymer used for a bottom film of a multilayer resist; and a copolymer for semiconductor lithography available by this process. In particular, the invention pertains to a preparation process of a copolymer for semiconductor lithography which does not contain a high molecular weight component having a molecular weight of 100,000 or greater (high polymer) and generates far fewer defects in resist patterns; and a copolymer for semiconductor lithography available by this process.
2. Background Art
In lithography for the fabrication of semiconductors, the formation of finer patterns has been requested with an increase in the degree of integration. For the formation of finer patterns, the shortening in the wavelength of an exposure light source is indispensable. Lithography with a krypton fluoride (KrF) excimer laser light (wavelength: 248 nm) is becoming mainstream now, while lithography with an argon fluoride (ArF) excimer laser light (wavelength: 193 nm) which can attain a line width of 100 nm or less will be industrialized in near future. A variety of lithographic technologies using radiation of a short wavelength such as a fluorine dimer (F2) excimer laser light (wavelength: 157 nm), extra ultra-violet radiation (EUV), X ray and electron beam are under development.
In such semiconductor lithography, various coating films such as resist film for the formation of patterns to be transferred to a substrate by utilizing a solubility change of a resist polymer in an alkaline developing solution caused by the action of an acid to the resist polymer, or films lying below or over the resist film are formed. Examples of the underlying film include an antireflective film for forming fine resist patterns precisely while suppressing a reflected light from a substrate; a flattening film to be used as an underlying film of a resist film, upon formation of a resist pattern over the substrate already having a pattern thereover, in order to flatten the surface of the substrate; and a bottom film of a multilayer resist to which a resist pattern is transferred by dry etching.
These coating films such as resist film are each formed by dissolving a copolymer for lithography having a function appropriate for each coating film and other additives in an organic solvent to prepare a coating solution, and applying the resulting solution to a substrate by spin coating method or the like, and removing the solvent by optional heating. The copolymer for lithography is required to have basic properties as a copolymer for coating film such as no inclusion of foreign matters disturbing the formation of fine patterns, as well as optical properties, chemical properties, coating properties and physical properties such as adhesion to a substrate or underlying film, all necessary for a resist film or antireflective film.
Resist polymers used for a resist film can be classified into a negative type whose solubility in an alkaline developing solution lowers by the action of an acid and a positive type whose solubility in an alkaline developing solution heightens by the action of an acid. Positive type resist polymers have, as essential components, a recurring unit having a structure in which a non-polar substituent is decomposed by an acid and a polar group soluble in an alkaline developing solution appears and another recurring unit having a polar group for heightening adhesion to a semiconductor substrate, and optionally a further recurring unit having a polar or non-polar substituent for regulating the solubility in a resist solvent or alkaline developing solution. As the recurring unit having a polar group for imparting the resist polymer with adhesion to a substrate, hydroxystyrenes are mainly used when a KrF excimer laser is used as an exposure light source. When an ArF excimer laser light is used as an exposure light source, use of polar-group-containing (meth)acrylates instead of hydroxystyrenes is investigated, because the latter ones absorb light having a wavelength of 193 nm.
As such a positive resist polymer for KrF system, known are copolymers using a (meth)acrylic acid type monomer and a styrene type monomer in combination (refer to, for example, Japanese Patent Laid-Open Nos. 45439/1984, 113667/1993, 209868/1995, and 65120/1999) and polymers having a hydroxystyrene protected, at a portion thereof, with acetal (refer to, for example, Japanese Patent Laid-Open Nos. 115440/1987, 219757/1992, 223860/1991 and 104251/1929), while as that for ArF system, known are co-polymers of a (meth)acrylic acid type monomer having a lactone structure (refer to, for example, Japanese Patent Laid-Open Nos. 73173/1997 and 239846/1998).
As the polymer used for an antireflective film, known are polymers obtained by copolymerizing an aromatic-nucleus-containing vinyl compound such as styrene, styrene derivative or anthracenylmethyl (meth)acrylate and an acrylamide derivative or a hydroxyl- or epoxy-containing vinyl compound, and optionally an alkyl (meth)acrylate (refer to, for example, Japanese Patent Laid-Open Nos. 313779/2000, 27810/2001, 192411/2001 and 226324/2001). As the polymer used for a flattening film, known are copolymers between a hydroxystyrene and a monomer such as styrene, alkyl (meth)acrylate or hydroxyalkyl (meth)acrylate (refer to, for example, Japanese Patent Laid-Open No. 57828/2003).
As resist patterns are becoming finer, more minute defects of resist patterns become a target of control and requirements for the performances of the above-described film forming polymers for semiconductor lithography become severe. Defects of resist patterns occur owing to the existence of foreign matters in the resist composition. Foreign matters in a resist solution were conventionally removed by using a filter of a micropore size or a filter having a zeta potential (refer to, for example, Japanese Patent Laid-Open Nos. 307263/1993 and 2001-350266) and foreign matters (contaminants) such as dust existing in the circumstance could be removed completely. Extremely minute, sparingly soluble substances however could not be completely removed by filtration and have been a great hindrance to the miniaturization of patterns formed by semiconductor lithography.
As a result of the investigation by the present inventors, it has been found that a trace amount of a high molecular weight component having a molecular weight of 100,000 or greater (which may hereinafter be called “high polymer”), as one of the extremely minute, sparingly soluble substances, has a serious influence on the formation of resist patterns. Described specifically, the solubility of a polymer generally depends on its molecular weight. High polymer molecules are sparingly soluble in a resist solvent or aqueous alkali. Even if it seems to be dissolved in the solvent or aqueous alkali, it is presumed to be a cause for defects when fine patterns are formed by lithography. In addition, when a high polymer exists, insoluble foreign matters (liquid-borne particles) grow with the high polymer as a nucleus with the passage of time during the storage of the resist solution and they appear as precipitates. There is therefore a high possibility of insoluble foreign matters being a cause for defects of resist patterns. There is accordingly an eager demand for the provision of a copolymer for semiconductor lithography in which an increase in liquid-borne particles is small during its storage period; and a preparation process of the copolymer.
In general, a film forming polymer for semiconductor lithography such as resist polymer can be prepared by radical solution polymerization of a raw material monomer. When a trace amount of a radical is generated before the temperature reaches the polymerization temperature, a radical concentration lower than the monomer concentration induces formation of a high polymer owing to the relationship as indicated by the below-described equation (1) and this is presumed to have serious influences as described above on the formation of resist patterns.Pn∝[M]/[R·]  (1)wherein, Pn represents the molecular weight of a polymer prepared, [M] represents a monomer concentration and [R·] represents a radical concentration.
In the case of so-called bulk polymerization in which polymerization is carried out after dissolving a monomer and a polymerization initiator in a solvent and then heating the resulting solution to a polymerization temperature, a high polymer tends to be formed during the retention time prior to heating or during the time until the temperature of the solution reaches the polymerization temperature owing to a radical concentration lower than the monomer concentration. A method of dissolving a monomer in a polymerization solvent and after the temperature reaches polymerization temperature by heating, adding a polymerization initiator to the resulting solution (refer to, for example, Japanese Patent Laid-Open No. 109153/2001) is known. This method is however accompanied with the drawback that during heating of the solution containing only the monomer, a high polymer tends to be formed owing to a radical generated in a trace amount due to impurities in the monomer solution. The so-called dropping polymerization method (refer to, for example, Japanese Patent Laid-Open Nos. 269754/1992 (Examples), 39444/1993 (Examples), 247386/1993 (Examples), 311110/1993 (Examples), 231538/1999 and International Patent Publication No. 99/50322) in which polymerization is effected by the dropwise addition of a solution, which has been obtained by dissolving a monomer and a polymerization initiator in a solvent, to a solvent heated to a polymerization temperature is known. Even this method is however accompanied with the drawback that a high polymer is formed owing to a trace amount of a radical generated during the retention time starting from mixing of the monomer with the polymerization initiator until the dropwise addition.
In radical polymerization reaction, by deaeration of a solvent or purification of a monomer, a radical trapping substance such as oxygen or polymerization inhibitor is often removed in advance before polymerization (refer to, for example, Japanese Patent Laid-Open No. 2003-221403). The investigation by the present inventors however has revealed that such an operation is causative of the generation of a high polymer.